1. Field of the Invention
The invention relates to a sheet delivery at a printing machine with a sheet brake formed of a brake roller, in which suction bodies are axially shiftably disposed on a carrier shaft extending transversely to the sheet transport direction; the peripheral surfaces of the suction bodies define sheet carrier surfaces with suction nozzles formed therein, to which there is briefly applied suction air in phase with the working cycle of the machine; the suction bodies are driven via the carrier shaft at a circumferential speed which is slower than the sheet transport speed of the arriving sheets.
2. Description of the Related Art
The device disclosed in German Petty Patent DE-GM 84 30 170 defines the pertinent genus. Known therefrom is a two-part sheet brake at the chain delivery of a printing machine. The sheet brake is formed of rotating, disk-shaped suction bodies, which are disposed at an axial distance from one another on a carrier shaft. The carrier shaft extends transversely with respect to the sheet transport device in front of the sheet stack in the delivery. Suction air control heads are provided for each of the suction bodies.
The suction bodies are axially adjustably mounted on the carrier shaft. The braking action on the sheets to be deposited is effected by a circumferential speed of the suction bodies which is slower than the transport speed of the sheet. The suction bodies mutually form a brake roller. Set at a slight advance relative to the arriving sheet, they briefly attract and hold the sheet at the moment of its release from the grippers at the chain delivery. Due to this, the sheet can be stretched by the suction roller and it is lowered onto the sheet stack.
A further sheet delivery is known from German Patent Document DE 32 20 798 C1. A suction roller is effective as a sheet brake. Continuous bores are formed at the periphery of the roller. The bores are continuously distributed in a left or right oriented helically spiralling line. The bores are briefly connected to the suction air via suction air feed members in phase with the working cycle of the machine at a respectively correct instance. Such a configuration is only effective in connection with a pre-disposed table on which the sheet is lightly blown at with an air stream from below and is thus weighted against the table surface by suction due to a hydro-dynamic paradox.
A suction roller in the sheet delivery of a printing machine is known from German published, non-prosecuted application DE 41 16 510 A1, wherein suction bodies are disposed on the periphery of a suction air-conducting central pipe. The suction bodies are communicatingly connectible to the central pipe via suction air conduits. These components are housed in a suction tube with a plurality of openings in its periphery.
All of the prior art sheet deliveries have in common that the structural expense for the construction of the braking device and, most importantly, for the adjustment of the suction bodies is substantial. Additionally, control of the suction air is complicated and expensive. The required number of connection hoses for the suction air disturbs format changes, especially in machines for larger formats. The adaptation of the machines to different print materials, print formats and print images is complicated and time consuming. The use of trailing edge-capturing devices for the sheets is impaired, particularly in small-format machines.